52 



WOOD AND FOREST. 



Woods do not vary nearly so much under compression as under 

 tension, the straight-grained conifers, like larch and longleaf pine, 

 being nearly as strong under compression as the hard woods, like 

 hickory and elm, which have entangled fibers, whereas the hard woods 

 are nearly twice as strong as the conifers under tension. 



Moisture has more effect on the strength of wood than any other 

 extrinsic condition. In sound wood under ordinary conditions, it 

 outweighs all other causes which affect strength. When thoroly sea- 

 soned, wood is two or three times stronger, both under compression 



and in bending, than when 



/ N green or water soaked. 6 



The tension or pulling 

 strength of wood is much af- 

 fected by the direction of the 

 grain, a cross-grained piece be- 

 ing only l/10th to l/20th as 

 strong as a straight-grained 

 piece. But under compression 

 there is not much difference; 

 so that if a timber is to be 

 subjected to cross strain, that 

 is the lower half under tension 

 and the upper half under com- 

 pression, a knot or other cross- 

 grained portion should be in the upper half. 



Strength also includes the ability to resist shear. This is called 

 "shearing strength." It is a measure of the adhesion of one part of 

 the wood to an adjoining part. Shearing is what takes place when 

 the portion of wood beyond a mortise near the end of a timber, 

 A B C D, Fig. 43, is forced out by the tenon. In this case it would 

 be shearing along the grain, sometimes called detrusion. The resist- 

 ance of the portion A B C D, i. e., its power of adhesion to the wood 

 adjacent to it on both sides, is its shearing strength. If the mortised 

 piece were forced downward until it broke off the tenon at the shoul- 

 der, that would be shearing across the grain. The shearing resistance 

 either with or across the grain is small compared with tension and 

 compression. Green wood shears much more easily than dry, be- 



Fig-. 43. Shearing- Strength is Measured 



by the Adhesion of the Portion A, B, C, 



D or to the Wood on both sides of it. 



6 See Forestry Bulletin No. 70, pp. 11, 12, and Forestry Circular No. 108. 



